How to Use a Hydrometer for Beer

Hydrometers are extremely important instruments for home brewers to become acquainted with: they allow amateur beersmiths to track the fermentation process and discern the alcohol by volume (ABV) level of their finished product.

Learning how to use a hydrometer for beer brewing is a skill that can improve the quality of the end product, take some of the mystery out of the ABV measurement, and enable more precise and professional measurement of several key aspects of homebrewed beer.

What is a hydrometer?​

Hydrometers are measurement instruments that are used in many different industries, including petroleum, chemical, pharmacology and, of course, alcohol production.

They are used to measure the specific gravity of liquids, and are typically long, weighted stem-like measuring scales that are able to float freely within liquids.

Specific gravity is a measure of how dense a liquid is; more precisely, it is a ratio of the density of the liquid to the density of water, which is constituted as having a value of 1.

When hydrometers are used in beer brewing, they are used for one purpose: to measure the attenuation occurring in the fermentation process.

Attenuation is the process by which yeast converts sugar to ethanol, or alcohol, making it a very important process for brewing! Liquid that has a higher specific gravity reading has more sugar in it, and is considered denser; for this reason, the specific gravity of a batch of beer, wine or mead descends as it ferments and its sugars are changed into ethyl alcohol.

How to read a hydrometer for beer​

First of all, most brewers do not insert the hydrometer into their actual batch—they siphon out a sample of the wort (the pre-yeast mash of grains, malt and water) and use the hydrometer to measure the specific gravity of that smaller amount.

Beer brewers, although they may measure specific gravity several times during the fermentation process, are most concerned with two readings: the original gravity, which is done before the pitching of the yeast, and the final gravity, which is done when fermentation is complete. These are the values that are used to compute the alcohol by volume level, the calculation of which we will look at more closely in below.

After you have poured the wort into a trial jar or similar graduated cylinder or bucket, filling up about 2/3 of the container, drop the hydrometer in the liquid.

The hydrometer will float in the liquid, and the measurement should be read from the lowest level of the liquid's surface. This is VERY important—the liquid on the sides of the jar or cylinder is impacted by tension and friction and will rise to a higher level than the liquid in the middle will. Reading a measurement from the sides will give you an inaccurate result.

Many home brewers just use hydrometer readings as guidelines, but if you are serious about exact accuracy, it is important to adjust the reading to take temperature into account.

Hydrometers are standardized at about 60 degrees Fahrenheit, or 15 degrees Celsius, so if the liquid is at a higher or lower temperature the final reading should be raised or lowered to reflect the temperature difference. How to Brew has an excellent and precise chart, but there should be one included in the packaging of your hydrometer as well.​

How to use a hydrometer to measure alcohol​

After obtaining the original gravity reading and the final gravity reading, there is a fairly simple equation that can accurately estimate the presence of alcohol (in terms of percentage) in the liquid.

To arrive at the ABV, simply subtract the final gravity amount from the original gravity amount, and multiply that number by 131.25.

For those among us who are not mathematically inclined, there are several excellent online calculators that should do the trick. Unless you have done something terribly wrong, the outcome of this equation should be a positive number somewhere between 3 and 10%, depending on the type of beer you are brewing.​

There are a few ways to know if something isn't quite right without calculating the ABV. A good rule of thumb is that the final gravity reading should be anywhere from one quarter to one fifth of the original gravity reading.

Most beers have original gravity measurements of anywhere from 1.045 to 1.060, and finish between 1.005 and 1.015.

A final gravity reading that is too high may indicate that there was a problem with the yeast preparation, or that not all of the sugars properly converted into alcohol.

Learning how to use a hydrometer for beer ABV measurement can help nip these issues in the bud at the beginning of the fermentation process.

Can hydrometers go bad?

A well-made hydrometer cannot go bad under normal circumstances, but its readings will be inaccurate if it has sustained a crack or is otherwise damaged. The weight at the end of the hydrometer can also become unattached, which damages the hydrometer's ability to properly float in water and correctly do its job.

It is also essential to clean the hydrometer once in a while, using a vinegar solution, in order to get rid of the film that will build up on the arm over time.

More often than not, presumed hydrometer errors are actually user errors.

Is the hydrometer sticking to the side of the container, causing an inaccurate reading? Has a liquid temperature above or below 15 degrees Celsius been properly accounted for? Have the bubbles or froth in the liquid been properly shaken off?

All of these questions can help save you money on expensive repairs or time spent measuring the same batch over and over again.

If you are convinced that your hydrometer is broken, or it continues to give you incorrect readings, simply purchase a new piece of equipment: hydrometers are extremely inexpensive and should last for years with proper care.

On the other hand, if the same issues continue to occur with a new instrument, it may be time for you to upgrade to a refractometer.

These incredible instruments are able to measure the specific gravity of a liquid by evaluating how it bends light. Refractometers are significantly more expensive than hydrometers, but they are widely reported to be both more trustworthy and more effective.

Either way, it is certainly best to learn how to read a hydrometer for beer before moving on to more advanced home brewing equipment.

The power of the hydrometer

Hydrometers are very useful tools, but it is important for beginner brewers not to rely too much on their readings. The most important tools for a brewer to use are his or her tongue and taste buds— if the hydrometer's specific gravity reading is lower or higher than expected, but the beer still tastes good, use the experience as a learning opportunity.

Hydrometers will not perform their function if they are not used correctly, so check out this great video to hone your technique and train yourself how to use a hydrometer for beer.

Add hydrometer expert to your brewing resume, and your batches will see a huge leap in quality and exactitude​.

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About the Author

Professional jockey Jose Sanchez always liked to kick back after a training session with a high-strung thoroughbred, but he'd never indulged in beer until, one day, the raven-haired owner of his most challenging mount slid him one from across the table. One sip of the ice-cold, frothy pale ale and Jose was a changed man. Now married to an heiress, Jose works as a sports journalist and collects cars.

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